I pretty much exclusively carry a SPAS-12 with a grip as my secondary. I use it as a primary in a lot of maps (like Derail, where I mix it with a Sniper rifle or AR w/ ACOG) it's death in the building.

The bad thing about most of the shotguns is if you miss the first shot or don't get a one shot kill you are pretty much screwed. But with the AA12 with Extended Mags you can clear out an entire building without ADS or reloading, just hold down the right trigger and you killed the entire team before they know what happened.